1. Field of the Invention
The present invention relates to a method of manufacturing thin can steel plate used for a tinplate, tin-free steel, and the like. More particularly, the invention relates to a method of manufacturing a higher-strength can steel plate having a smaller thickness and better workability compared to conventional can steel plates.
2. Description of the Related Art
Can steel plates, in particular, beverage can steel plates, are becoming thinner with a view to saving resources and achieving weight reduction. Improvements are being made to make the can steel plates thinner-walled. There is also a demand for good workability in the application of such steel plates to two-piece cans.
Conventionally, for example, a box-annealing material having a thickness of approximately 0.33 mm or greater and a tempering degree of approximately T1 (a strength (TS) of from approximately 32 to 33 kgf/mm.sup.2) is used for DI two-piece cans. The thickness of such a material has recently been gaged down to 0.29 mm and even to 0.25 mm or smaller. Along with the downsizing of the material, there follows an increase in the use of high-strength materials having a tempering degree of T2.5 (a strength TS of approximately 37 kgf/mm.sup.2), and high-strength materials even having a tempering degree of from T3 to T4 (a strength TS of from approximately 38 to 39 kgf/mm.sup.2).
Further, since deep drawing is performed on the two-piece cans in the manufacturing process, there is a demand for a large degree of average r and a small degree of .DELTA.r. For example, there is a demand that a DI can steel plate should have an average r of 1.3 or greater and .DELTA.r of 0.3 or smaller. A small degree of .DELTA.r is demanded with a view to suppress earing during deep drawing for improving yield of produced can and to avoid breaking the earing during ironing performed on the coarse form of the can and during a subsequent process of removing the can from a punch.
Although many proposals have been made for a method of manufacturing a can steel plate, no proposal meets all the requirements described above.
For example, Japanese Patent Laid-Open No. 2-118027 discloses a method of manufacturing a can steel plate having good workability. This method is employed whereby the so-called extremely-low carbon steel slab having a predetermined composition is subjected to hot rolling, cold rolling, and acid pickling according to a conventional procedure, being followed by cold rolling under a rolling reduction ratio of from 85 to 90% to obtain hot rolled steel strip. Subsequently, the resultant strip is subjected to continuous annealing and further to temper rolling under a rolling reduction ratio of from 15 to 45%, thereby strengthening the steel.
However, the foregoing method presents the following problems. Since extremely-low carbon steel is used as a material, it is necessary to perform temper rolling under a considerably high reduction ratio, subsequent to the continuous annealing, in order to obtain a high-strength steel plate. This lowers productivity.
A proposal which was made to increase the strength of a can steel plate is disclosed in, for example, Japanese Patent Laid-Open No. 2-118025. Under this method N is added to the material of a steel, and the temper rolling is further performed after annealing, thereby increasing the strength of the steel.
However, the steel plate obtained by this method cannot meet the conditions of good workability and having a small planar anisotropy (.DELTA.r), which are required for manufacturing a two-piece can having a large reduction ratio.
A method of utilizing texture controlling technique by precipitating AlN during annealing is well known as a method of ensuring good workability. However, this method presents the following problems. Since AlN is precipitated during annealing, a comparatively slow heating speed is required. This typically necessitates the employment of a box-annealing process, and thus it is very unlikely to be able to provide a cost-effective continuous annealing method.
Further, Japanese Patent Laid-Open No. 63-230848 discloses a method of manufacturing a steel plate having good workability through the use of texture controlling technique by means of the precipitation of AlN during the continuous annealing process. This method is employed as follows. A steel having a composition of C.ltoreq.0.003%, Mn=0.09-0.8%, sol.Al=0.06-0.12%, and N=0.005-0.011% is used. It is subjected to hot rolling and is then coiled at a temperature of 560.degree. C. or lower. Subsequently, it is subjected to cold rolling and is continuously annealed under the conditions of an average temperature rise speed of from 1.degree. to 20.degree. C./s in a range of from 400.degree. to 700.degree. C. and a maximum heating temperature of from 700.degree. to 900.degree. C. This process is intended to ensure good workability.
However, this method requires the content of a large amount of Al as much as 0.06% or higher. This promotes the precipitation of AlN during hot rolling, and the amount of precipitated AlN varies, making it difficult to control the amount of dissolved N, prior to continuous annealing. This further makes it difficult to control the amount of AlN which should be precipitated in the process of continuous annealing, thereby making a variation in the material quality wider. Additionally, a large amount of Al content makes the product expensive.